1. Field of the Invention
The invention relates to a semiconductor memory device such as a Not AND (NAND) flash memory, and particularly relates to storing and reading of information with a high security level.
2. Description of Related Art
A NAND flash memory has a memory cell array, and the memory cell array includes a NAND string serially connecting a plurality of memory cells. Compared with a NOT OR (NOR) flash memory, the NAND flash memory is capable of realizing a memory cell array having a high density, and is thus suitable for data storage with a large capacity for image or music data. In addition to such purpose, the NAND flash memory also functions as the memory providing a boot code when an electronic apparatus or system is booting. The boot code is data for booting an electronic apparatus at a host side or an operating system of the system.
FIGS. 1A and 1B are diagrams illustrating a system configuration of a conventional semiconductor memory capable of outputting the boot code to a host system. As shown in FIG. 1B, a semiconductor memory 10 is connected to a host device 30 through a bus and so on. As shown in FIG. 1A, the semiconductor memory 10 has an input/output pin 12 for inputting and outputting data with the host device 30, a memory controller 14, and a memory part 16. The memory controller 14 includes a host interface 20 for data transmission with the host device 30 through the input/output pin 12, a memory interface 22 for data transmission with the memory part 16, a micro-processing unit (MPU) 24 controlling the data transmission, and a read only memory (ROM) 26 and a random access memory 28 for storing programs or data. The memory part 16 includes two NAND flash memory chips, for example. Also, as shown in FIG. 1B, the memory part 16 includes a physical access region 16A accessible with a physical address and a logical access region 16B accessible with a logical address. The boot code of the host device 30 is stored in the physical access region 16A. The boot code is data for booting an operating system of the host device 30. Based on the configuration, under the circumstance that the host device 30 only supports physical access, the boot code may be provided to the host device 30 (Patent Document 1).
Conventional Art
Patent Document 1: Japan Patent Publication No. 2009-175877.
In a host system using the NAND flash memory as the memory for storing the boot code, the boot code may be read from the flash memory when the system is booting or when power is supplied, so as to boot the system. Besides, the following applications are included in addition to the boot code: classified important information having a high security level stored in advance, wherein the information is read and used for system control. For example, the application may be as follows: storing in advance a system-specific security code or decryption key in the flash memory. If the security code cannot be read, the memory system is unable to operate normally. More specifically, the application may be allowed to boot the operating system or software stored in the flash memory by matching of the security code. Besides, as another example, another application may be as follows: storing in advance highly private personal information in the flash memory and using the personal information in a specific program.
However, storing in advance the information with a high security level in the flash memory and reading the information during an operation renders the following issues. The risk of information leakage increases if important information such as security code or private information is read multiple times. For example, sometimes the information may be read or replicated without authorization by a non-specified system or a hostile user. Thus, the protection may not be deemed as sufficient. Besides, while it is possible to set such information to be accessible only when booting, similar to the boot code, the time limitation on using the information may be too restrictive.